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I've been working on a blog where I take fictional planets and see how they might work in the real world. I recently did Qo'noS, the Klingon homeworld from Star Trek, and was a bit stumped about one fact - the sky is "green and noxious".

I took a punt and said maybe the atmosphere had a lot of chlorine gas in it, but I don't think that's a fool-proof assertion...

I don't know that much about atmospheric chemistry, or the physics of colour for that matter. Any thoughts on what makes their atmosphere green?

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    Since you're looking for real-world physics to help with your answer, I'd suggest asking the question on physics.stackexchange.com. Questions seeking real-world answers usually get flagged as off-topic here. – Tim Jul 18 '17 at 1:34
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    My guess would be "industrial pollution". – Harry Johnston Jul 18 '17 at 1:39
  • Chlorine gas in the atmosphere would make it too poisonous for humans. So much for Picard visiting Qo'nos twice every season. – RichS Jul 19 '17 at 0:06
  • Canon describes the atmosphere as 'noxious'. Of course, oxygen is pretty noxious too - it all depends on concentration. In my article I imagined that there's a concentration high enough to colour the atmosphere but low enough that it doesn't cause problems to breathers at ground level. – Rohan S Byrne Jul 19 '17 at 2:45
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    This question seems to be asking an in-universe question and is therefore not off-topic. – Edlothiad Aug 15 '17 at 21:17
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It could be green if their sun is a red giant and their atmosphere is similar to ours. It would range from yellow to green.

  • You could supply the maths that supports this. – Gallifreyan Jul 18 '17 at 18:41
  • @Gallifreyan You mean physics? This question has answers that support endoric's answer, although they don't have actualy calculations. Not sure that's exactly needed though. – Arthur Dent Jul 18 '17 at 18:44
  • Are you sure that planets similar to Earth orbiting red giant stars have green atmospheres? Where do you get that assumption? – RichS Jul 19 '17 at 0:05
  • Honestly I would just have to chalk it up to a sort of general knowledge in my personal life. I have no math or solid facts, just one of those things in a discussion with far smarter people about what earth would be like if we had a red giant for a sun to begin with and we're still in the habitable zone. – endoric briggs Jul 19 '17 at 1:57
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    I'm fairly sure this wouldn't work no matter what colour the sun is, for the same reason that the sky goes from blue to yellow to red in the evening without ever being green at any point. Also consider that our sun is about as green as stars get, yet appears yellow in our atmosphere and white when seen from space. – Rohan S Byrne Jul 19 '17 at 2:44
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Expanding from endorin briggs answer with some thoroughly amateur arm-chair physics and assumptions. For reference, the theory is that a Red Giant could account for Quo'Nos having a green sky.

According to Wikipedia's article on Diffuse Sky Radiation, it seems our sky shows light blue because short wavelengths (blue-ish) scatter more than longer wavelengths (reddish). This is apparently why the sky appears more yellow around the sun, as the light is traveling more directly towards you, and thus is subject to less scattering.

Our sun has a pretty broad color spectrum, although it cuts off in the violets, otherwise our sky would probably look much more purple.

The emissions from our sun peak in the "visible spectrum", which includes our standard colors. A red giant's emissions peak in the infrared, but would also pass into the visible part of the spectrum, just at much lower intensity, and probably tapering out with very low concentrations into blue/violet. If Quo'nos orbited a Red Giant, the light would have less radiation in the blue or violet wavelengths than it did in the green wavelengths.

Now, it would still have much more red light, but depending on how the radiation curve looked, we could conceivably end up with enough radiation in the green range to scatter more (especially if Quo'nos has a thicker atmosphere) green light, but have very little blue light left to overpower it. Red scatters the least of these, so the green could theoretically be the dominant color. If this were true we would also expect to see a strong reddish halo or region in the sky around the sun itself. Interestingly, the horizon might look blueish at noon on Quo'Nos, as that's the location where the most blue/violet light scattering would be visible.

Now, if we step back to the TV show, we can see that the Klingon's favor deep red lighting in general, and tend to have darker (to human eyes) illumination aboard their ships/stations. This could indicate that they see further into the infrared spectrum than humans. If so, this would support them evolving as predators on a planet orbiting a Red-Giant, where their eyes would be adapted to the different color spectrum.

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